Molecular field theory for biaxial nematics formed from liquid crystal dimers and inhibited by the twist-bend nematic

Abstract

Liquid crystal dimers with odd spacers are good candidates as materials for biaxial nematic phases (N_B). The dimers are flexible molecules sustaining biaxial conformations, and couplings between the conformational and orientational distributions could be expected to stabilise N_B. We apply a molecular field theory for flexible molecules developed elsewhere to study a simple system made up of dimers composed of two cylindrically symmetric mesogenic groups. Our model allows for two idealised conformations: one linear and one bent at a tetrahedral angle. For a restricted set of chain lengths, the model predicts a first-order reentrant phase transition from the N_B phase into a low temperature uniaxial nematic phase (N_U). However the formation of the biaxial nematic could be blocked by the appearance of a twist-bent nematic.